The aim of the paper is to investigate the usefulness of different illustration formats in online English learners' dictionaries for the accuracy and speed of meaning comprehension as well as immediate and delayed retention. In a controlled online experiment, the meaning of selected English words and phrases had to be explained with the help of purpose-built monolingual dictionary entries. Four experimental conditions were created, which reflected the presence and format of illustrations in the entries: color pictures, greyscale pictures, line drawings, no illustrations. Meaning retention was checked immediately after exposure and two weeks later. The results show that it is worthwhile to include illustrations in online learners' dictionaries and suggest the most beneficial illustration formats. Line drawings prove the most recommendable; they considerably improve meaning comprehension, reduce reception time, and stimulate the best immediate and delayed retention. Color pictures emerge as the second best. They produce results comparable with those for line drawings, except they do not help so much to remember meaning in the long run. Entries with greyscale pictures are the least recommendable. They do not contribute more to meaning comprehension and delayed retention. Yet, they even shorten reception time and help users to remember more words immediately after exposure.